In addition to protection of a squirrel cage motor based on current measurement, thermistor sensors can be used to protect windings against overheating, for example when the ambient temperature rises or motor cooling is insufficient for any reason. When a squirrel cage motor is used in an explosive atmosphere (Exe), the regulations require that the motor must be equipped with a temperature measurement circuit and the measurement circuit of the thermistor sensor must be galvanically isolated from the other electronic circuits of the system. In practical three-phase motors, three thermistor sensors are connected in series, one for the stator winding of each phase.
One conventional solution is based on AC technology, including an astable multivibrator operating on a selected frequency and a galvanic connection between the measurement circuit and the device electronics. Galvanic isolation is implemented by means of an isolation transformer between the thermistor sensor and measurement circuit. A disadvantage of this solution is the non-linearity of the isolation transformer and its poor response, particularly at low sensor resistance values, which makes it difficult to indicate a short circuit. Another disadvantage is the effect of capacitance in the sensor circuit conductors, which is summed to the measurement circuit through the isolation transformer in addition to the actual effect of the sensor resistance. Galvanic isolation by an isolation transformer is simple to implement, and the solution is inexpensive.
Another conventional solution is the use of a linear opto-isolator, but this causes problems due to the high price of the component and large differences in tolerance between the individual components, which makes the implementation of calibration for the measurement circuit complicated. Reliability and stability in long-term use are poor as well. The advantage of this solution is good linearity over a wide range of resistance and a rapid operating response.
An electric motor manufacturer most typically chooses a PTC thermistor sensor, the resistance of which changes very sharply just below the maximum temperature allowed for the winding insulation. A prior art solution based on this compares the resistance value of the thermistor sensor with a fixed threshold value. Crossover of the threshold value is indicated either by a change in the current load of the isolation transformer or by an opto-isolator. The solution can be inexpensive, but its disadvantage is the ON/OFF type status information; no information is available on the state of change in the resistance value—for example, whether it is approaching the threshold value slowly or rapidly. No indication of a short circuit fault in the sensor circuit is provided either.
There is also an integrated circuit in the market, Maxim Max 6691, that allows the connection of four PTC or NTC sensors. The chip converts the resistance value of each sensor to one Pulse Width Modulated (PWM) pulse; however, the range of the pulse ratio is quite narrow. The PWM pulse ratio decreases with increasing resistance, so the chip is most easily applicable to a NTC sensor. The linear operation of the circuit makes it difficult to distinguish a short circuit from small values of resistance. The measurement range can be adjusted using a resistance connected in series with the sensor, but the usable measurement range in each case is too restricted with regard to an application for measuring the temperature of electric motor windings and protecting them. There is no second-source manufacturer of the chip, which means that it will bind the user to a single component supplier.
There is another integrated circuit, Smartec SMT 160-30, with an internal temperature sensor and PWM output. Due to its physical size and limited operating temperature range, the chip is not applicable to the protection of electric motor windings. The PWM frequency of the chip output is 1 . . . 4 kHz, which also sets excessively tight requirements for the interface component and the circuits for analysing the measurement result.